1. Field of the Invention
This invention relates to optical devices, and more particularly, to wavelength selectable light sources and tunable lasers.
2. Description of the Related Art
Wavelength selectable light sources and tunable lasers are well known in the art. Wavelength selectable light sources are often used in place of a tunable laser, or in conjunction with laser tuning. Tunable lasers function as wavelength selectable light sources by varying a combination of temperature, current, or other suitable input to the device, such that the wavelength out put from the tunable laser can be selected. For this reason, tunable laser approaches and wavelength selectable laser approaches will be described herein and the phrase “wavelength selectable light source” will be used herein to refer to both.
The following references, which are incorporated by reference herein, provide a good review of the technology.                (1) L. A. Coldren, “Monolithic Tunable Diode Lasers,” IEEE Journal on Selected Topics in Quantum Electronics, Vol. 6, No. 6, pp. 988–999, 2000.        (2) C. J. Chang-Hasnain, “Tunable VCSEL,” IEEE Journal on Selected Topics in Quantum Electronics, Vol. 6, No. 6, pp. 978–987, 2000.        (3) B. Pezeshki, et al., “20-mW Widely Tunable Laser Module Using DFB Array and MEMS Selection,” IEEE Photonics Technology Letters, Vol. 14, No. 10, pp. 1457–1459, 2002.        
Generally, wavelength selectable light sources can be classified into four main categories:                (1) In-plane tunable lasers, including sample grating lasers, and vertical grating coupler filter lasers,        (2) External cavity lasers having movable gratings and/or mirrors,        (3) Tunable vertical-cavity surface-emitting lasers (VCSELs) having deformable mirror devices, and        (4) In-plane laser arrays combined using multiplexers or linearly-cascading the lasers.        
The first two categories of devices operate primarily on the principle of varying the wavelength for which strong feedback occurs into the active region. The third category operates on the principle of varying the cavity length of the laser (this principle is present to a lesser extent in the operation of the first two categories of devices as well). The last category utilizes one-dimensional (1-D) in-plane laser arrays. Each element of the array can be a tunable laser or a fixed-wavelength laser. The lasers can be linearly cascaded such that they share the same waveguide, or they can be combined using a multiplexer.
The Pezeshki, et al. reference cited above is an example of a 1-D distributed feedback (DFB) laser array that utilizes a single movable mirror and a lens as the multiplexing technique. This technique works well for small device array dimensions, such as the 120 micrometer array width used in the reference. For large device array dimensions, large optical elements and large tilt angle movable mirrors are required. This poses technical problems that could include difficulties, such as fabricating a large flat movable mirror, slow response time of the large mirror, aberrations in the optics, and astigmatism in the lens. These difficulties affect the usefulness of this technique when larger array dimensions are considered.
In technologies that utilize a single active region structure as the gain medium, the tuning range is fundamentally limited to less than 100 nm, and typically the useful range is less than 40 or 50 nm. Even technologies that can use multiple device active regions for gain at different wavelengths, such as linear cascade and multiplexed in-plane lasers, are limited in their wavelength range due to difficulties in scaling to a high number of devices and in coupling the power from all the devices efficiently into a single fiber. Consequently, there is a need in the art for improved wavelength selectable light sources.